Implantable cardiac stimulating device with optimized demand

ABSTRACT

It is important in cardiac pacing devices and systems to achieve efficient power utilization and conservation to extend the life of the battery cells, thereby extending the intervals between invasive medical procedures to replace components in the cardiac pacing system. A device and method are provided. The cardiac pacing device comprises a battery, a discrete time switched capacitor pacing power supply comprising a charge transfer capacitor bank comprising at least two capacitors, and a pace output supply capacitor which can discharge current to the tissue of a patient. A pacing supply design has a multiplicity of battery voltage multiplication factors and operating frequency settings. The pacing supply, voltage multiplier settings and operating frequency are automatically adjusted to compensate for changing pace output settings, load, cardiac cycle rate, and/or battery condition. The pacing supply voltage multiplier setting and frequency are varied over the useful life of the battery so that on average, from a beginning of life period for the battery to an end of life period for the battery, the voltage multiplier and the switching frequency increase. In the pacing device, the voltage multiplier and the switching frequency rate is determined by a reading of the rate of charging or charge time of a storage capacitor with the pacing device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/302,853, filed on Apr. 30, 1999, the specification of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to an improved cardiac rhythmmanagement apparatus, and more particularly to an implantable cardiacstimulating device incorporating means for leveling and/or optimizingthe pulsatile pacing capacitor charging current demand from an internalbattery power source.

BACKGROUND

Heart disease is a major cause of deaths in the United States and inother industrialized nations. One well-known treatment approach utilizesan implantable cardiac pacing device, through which relatively mildperiodic electrical impulses are applied to epicardial or endocardialtissue as necessary to maintain normal sinus rhythm. More recently,cardioversion/defibrillation devices have been developed to counteracttachyarrhythmias (rapid disturbances in cardiac electrical activity). Inparticular, the conditions of ventricular tachycardia, ventricularflutter and ventricular fibrillation are widely believed to be theprimary cause of sudden deaths associated with heart disease.Defibrillation devices also are utilized to counteract atrialtachyarrhythmic conditions, although such conditions are not consideredlife threatening unless they lead to a rapid ventricular disturbance orunless persistent enough to cause blood clots to form within the atriumbecause of pooling caused by poor atrial ejection.

Tachyarrhythmic conditions frequently can be corrected by applyingrelatively high energy electrical shocks to the heart, a technique oftenreferred to as cardioversion. Cardioversion devices include implantableelectronic standby defibrillators which, in response to the detection ofan abnormally rapid cardiac rhythm, discharge sufficient energy throughelectrodes connected to the heart to depolarize and restore the heart tonormal cardiac rhythm.

Cardioversion/defibrillation devices frequently include epicardiallyimplanted electrodes. The surgical procedure required for implantation,i.e., thoracic surgery such as a median sternotomy or thoracotomy, ishighly invasive and presents significant risks to the patient. It ishighly desirable therefore to make every effort to reduce the number oftimes that invasive surgery is needed for a patient by having the lifeof each implanted device extended as long as possible. For this reason,extensive design and programming efforts are used to maximize the lifeof battery cells associated with cardiac pacing devices.

Electrodes implanted in the body for electrical cardioversion ordefibrillation of the heart are well known. More specifically,electrodes implanted in or about the heart have been used to reverse(i.e., defibrillate or cardiovert) certain life-threatening cardiacarrhythmias, where electrical energy is applied to the heart via theelectrodes to return the heart to normal sinus rhythm. See, for example,U.S. Pat. No. 4,291,707 to Heilman, relating to a planar patchdefibrillation electrode.

The Heilman patent specifically discloses an implantable cardiacelectrode comprised of a planar conductive material insulated completelyon one side and partially on its other side. Apertures are providedaround the insulated perimeter of the partially insulated side of theelectrode to provide for efficient and uniform energy transfer to theheart tissue by eliminating the so called “edge-effect”.

The amount of energy delivered by the electrodes to the heart duringdefibrillation (or cardioversion) depends on the placement of theelectrodes and the ability of the electrodes to distribute the energyuniformly throughout a major portion of the heart. This energy is calledthe defibrillation or cardioversion energy.

U.S. Pat. No. 5,312,442 relates to the art of implantable cardiacdefibrillators, and in particular, is related to an energy dissipationresistor capable of efficiently and reliably dissipating energy storedin the capacitor(s) of an implantable cardiac defibrillator. It is oftenthe desire to dissipate electrical energy stored in defibrillatorcapacitors, rather than discharging to the heart, by diverting thecapacitor voltage to an internal resistor. This is commonly referred toas an “internal dump”. See, for example, U.S. Pat. Nos. 4,316,472 and4,488,555 to Imran and Mirowski, respectively, where internal loadresistors are shown. Presently, conventional resistive elements, such ascarbon or ceramic resistors, and the like are used to dissipate theenergy. Such conventional resistive elements tend to be large and bulky,and therefore difficult to package, often requiring extensive incominginspection processes to assure that a desired reliability is achieved.In addition, although conventional resistive elements have provengenerally effective in practice, worst case testing with multiple shockand internal dump episodes has resulted in heating of the resistiveelement and occasional destruction thereof.

For example, U.S. patent application Ser. No. 08/550,835, filed on Oct.31, 1996, in the name of M. Brook and titled “Power Management Systemfor an Implantable Device”, shows a process for managing a power source,the power source having an output voltage, comprising the steps of:

periodically switching a load across the power source using a switch;

monitoring the output voltage of the power source using a dedicatedvoltage monitoring device and current monitoring device;

if the current monitoring device detects a current equal to or greaterthan a predetermined current threshold, opening the switch for oneswitching period; and

if the output voltage is less than a selected threshold voltage, openingthe switch until the output voltage is greater than the selectedthreshold voltage. This process is designed to and effectively doesreduce the overall power utilization of a pacing system and can act toextend the life of a digitally charged power supply.

Cardiac pacing systems can tend to be quite active, with many differentfunctions (sensing, pacing, charging, defibrillating, etc.) beingprovided by a single complex system. The activity of the system providesa significant drain on the usually single implanted battery which powersthe entire system. Because the replacement of a battery is an invasiveprocedure which should be minimized in its frequency, anything which canbe done to optimize the performance of an implanted pacing system or inanyway reduce the drain on the power supply to extend the life of thebattery is a desirable goal.

Cardiac pacing systems are almost exclusively electronic, with no movingparts, so energy usage is already at relatively efficient rates. Anysavings in energy usage is therefore very significant in extending thelife of the battery in the pacing unit and in avoiding any invasivemedical procedures. There are three general areas in which powerutilization may be controlled: a) assuring that no work in performedwhen the inherent performance of the heart occurs, b) optimization ofthe function and power utilization of the components themselves, and c)programming of the pacing device to control the output of energy. Anynew variation within these areas or new procedures which can be executedto reduce energy usage and prolong the life of the battery are highlydesirable, in addition to the specific means shown in U.S. patentapplication Ser. No. 08/550,835 shown above.

Those skilled in the art will appreciate that the body has severalmechanisms designed to adjust to metabolic demand by modifying cardiacoutput and, ultimately, oxygenation of tissue. The cardiac output may beincreased or decreased by adjustment in sinus rate or stroke volume.Recognizing this fact, it has been possible to monitor metabolic demandby monitoring the sinus rate, the stroke volume and/or other activitieswhich indicate different levels of use required by the patient's system.Both the monitoring of the patient's system and the exercise of thesevariations in pacing frequency require additional energy utilization andenergy drain on the battery cell, and system must be put into effect toallow for these functions to be provided without shortening the life ofthe standard batteries used within the field.

U.S. Pat. No. 5,397,342 relates to body implantable tissue stimulationelectrodes, e.g., for cardiac pacing or cardioversion/defibrillation,and more particularly to the deployment and implantation of suchelectrodes.

Examples of epicardial defibrillation electrodes are found in U.S. Pat.No. 4,567,900 (Moore), U.S. Pat. No. 4,291,707 (Heilman et al.), andU.S. Pat. No. 4,860,769 (Fogarty et al.). A pair of differently biased(e.g., oppositely polarized) epicardial electrodes can be employed, asshown in Moore. Alternatively, the Heilman patent discloses anintravenously inserted endocardial electrode arrangement in combinationwith a patch electrode positioned near the left ventricular apex.

U.S. Pat. No. 4,270,549 (Heilman) describes a technique for insertingand placing defibrillation electrodes, involving intravenous insertionof an endocardial electrode in combination with a patch electrodeinserted through a skin incision and through a tunnel created inside thethorax and outside the pleural cavity. Alternatively, U.S. Pat. No.4,865,037 (Chin et al.) discloses a technique for inserting separateelectrodes into the intrapericardial space through catheters. Anincision is formed in the upper abdominal wall. Then, tissues betweenthe incision and pericardium are separated, and an incision is then madein the pericardium. A cannula containing a defibrillation electrode isinserted through these incisions, to enable positioning of the electrodein the pericardium. A second cannula containing a second electrode isinserted on the opposite side of the heart, in the same manner.

A problem, particularly with patch electrodes, is the current densitygradient, i.e., maximum current density regions at the patch periphery.Current density gradients reduce the efficacy of the electrode, in termsof the ratio of useful cardioversion/defibrillation energy as comparedto required pulse generator output energy. Alternative energy conservingmeans are desirable to overcome or balance these types of inefficienciesin cardiac pacing devices.

With the criticality of extending the life of battery cells and inmaintaining a potentially large number of functions within the pacingdevice, any modification which can measurably extend the life of thebattery can be advantageous to a cardiac pacing system.

To minimize the size of an implantable pacing device, the power sourceis usually a single battery cell for a brady pacemaker, and one orperhaps two cells of different chemistry types for an implantabledefibrillator. Different battery types may be used within the sameimplantable defibrillator, one optimized for low-power high energydensity for powering the internal circuitry of the device, and aseparate high-power battery to provide the infrequent high energycharging of the defibrillator high output voltage capacitors.

The open circuit output voltage of these battery power sources aretypically very low (e.g. 2.0˜3.5V) because the batteries are oftencomposed of a single cell. Therefore, fundamental to cardiac pacingdevices and defibrillator design is the need for step-up voltage powersupply circuitry. Defibrillators require the battery voltage to bestepped up to 100's of volts to charge the defibrillator outputcapacitors. Even brady-cardia pacing can require pace pulse amplitudeslarger than can be delivered from the battery, the maximum programmableoutput amplitudes varies according to manufacture but is typicallybetween 4.5 to 7.0V, because occasionally these higher pace amplitudesare required to properly depolarize the heart depending upon pacing leadtype, placement, and health of the patient's heart.

There are many known circuit topologies for generating step-up as wellas step-down voltages, as indicated by the representative reference listbelow:

K. Asano, “Voltage Dropping Circuit”, U.S. Pat. No. 4,205,369, May 27,1980.

J. F. Dickson, “On-chip high -voltage generation in MNOS Ics using animproved voltage multiplier technique”, IEEE J. of Solid-State Circuits,vol. SC-11, pp. 374-378, June 1976.

R. Gregorian and G. C. Temes, “Analog MOS Ics”, Wiley, 1986, pp 463-464.

I. Harada et al., “Characteristics analysis of Fibonacci-type SCtransformer”, IEICE Trans. Fundamentals, vol. E75-A, no. 6, June 1992,pp 655-662.

F. Krummenacher et al., “Higher sampling rates in SC circuits by on-chipclock voltage multiplication, Proc. ESSCIRC, pp. 123-6, September 1983.

D. H. Oto et al., “High-voltage regulation and process considerationsfor high-density 5V-only EEPROMs”, IEEE J. of Solid-State Circuits, vol.SC-18, pp. 532-538, October 1983.

J. C. Ryan, K. C. Carroll and B. D. Pless, “A four-chip implantabledefibrillator/pacemaker chipset,” Proc. IEEE 1989 Custom IC Conf., pp.7.6.1-7.6.4, May 1989.

S. Singer, “Inductance-less up dc-dc converter”, IEEE J. of Solid-StateCircuits, vol.

SC-17, pp. 778-781, August 1982.

D. R. Squires, “Monolithic Voltage Divider”, U.S. Pat. No. 4,433,282,Feb. 21, 1984.

F. Suzuki and S. Ichikawa, “DC-to-DC Voltage Converter”, U.S. Pat. No.4,451,743, May 29, 1984.

U. Tietze and Ch. Schenk, Electronic Circuits, Sringer-Verlog, Berlin,1991.

F. Ueno, T. Inone and T. Umeno, “Analysis and application of SCtransforms by formulation”, Electronics and Comm. In Japan, Pt. 2, vol.73, no. 9, 1990, pp. 91-103.

J. S. Witters et al., “Analysis and modeling of on-chip high voltagegenerators circuits for use in EEPROM circuits,” IEEE J. of Solid-StateCircuits, vol. 23, pp. 1372-1380, October 1989.

D. Wayne et al., “A single-chip hearing aid with IV SC filters”, Proc.IEEE 1992 CICC, pp. 7.5.1-7.5.4, May 1992.

BRIEF DESCRIPTION OF THE INVENTION

Some switched power supply designs use both inductors and capacitorenergy storage elements, while others utilize capacitor only.Inductorless approaches are commonly employed by brady pacemakers torecharge the pacing output capacitor. These are typically switchedcapacitor designs which utilize one or more charge transfer capacitors,a set of interconnecting switches and non-overlapping digital clocksignals that are used to coordinates charge extracted from the batteryon one timing phase (the fill phase) and then deposited on the pacingoutput supply capacitor on a separate timing phase (the dump phase).Depending upon on the number of charge transfer capacitors and theswitching configuration provided by the design, a multiplicity ofvoltage multiplication can be achieved from the same circuit dependingupon setting.

For a brady pacing supply application, voltage multiplication settingsof (0.5x, 0.66x, 1.0x, 1.5x, 2.0x, 3.0x), can be achieved using just twocharge transfer capacitors, the battery which provides the input chargeto the supply, and the pace output capacitor which is charged inanticipation to the next pace output pulse to be delivered to the heart.

Continual semiconductor technology and circuit design improvements havereduced internal operating currents making pacing current supply designand control increasingly important to extend operating life, and reducethe size of the implanted device.

Optimal control of the voltage multiplier setting can have verysignificant effects on device longevity as the current transfer ratio,the ratio of the output charge delivered to the pace output supply toinput charge taken from the battery of the switched-C pacing supply isthe inverse of the voltage multiplication ratio. Stated another way, forevery microcoulomb of charge delivered to the output pacing supply at amultiplier setting of Kx, Kx times that amount of charge is removed fromthe battery.

Lead technology improvements now make capture thresholds of about 1.0Vtypical possible. Lower capture threshold leads, combined with automaticcardiac depolarization confirmation and threshold measurement systems(i.e. Autocapture) within the Brady pacemaker allow capture to bemaintained at very small “safety” margins, the excess voltage above themeasured capture threshold. This allows the pace output setting to beset just slightly above the capture threshold of the lead.

In this situation, pace output current draw from the battery can bereduced dramatically, by using voltage multiplication factors less than1.0x. For example, the terminal voltage of Lithium Iodine battery atbeginning of life will be very close to 2.8V. Therefore battery currentrequired for the pacing output supply can be reduced by half byincluding and using a 0.5x voltage multiplier setting rather than the1.0x voltage multiplier setting in the pacing supply. In this settingthe pacing supply can achieve an output voltage of up to 1.4V and willdo so at half the battery charge as the 1.0x setting. Previous designshave managed the programming of the pacing system with fixed constantslook-up tables which allow the determination of the appropriate voltagemultiplier setting Kx, given the desired pace amplitude setting.Typically, the relationship between pace amplitude and multipliersetting in the table-look-up is constant throughout the pulse generatorlife, and is calculated considering worst case operating conditions.Under worst case conditions, the multiplier setting must be chosenhigher to compensate for low battery terminal voltage at end-of-life,even if a higher efficient, lower multiplier setting may be sufficientto recharge the pacing output capacitor earlier in life when the batteryvoltage is higher.

This present invention achieves novel current reduction and maximizesbattery life by having the implanted device continually monitor andselect the minimum possible voltage multiplier settings and thefrequency of the switched capacitor timing clock as a function of thepacing output capacitor charge time (or alternatively charging rate) andbattery condition.

The present invention relates to a cardiac pacing device with apulsatile pacing output. The nature of the pulsatile output allows theopportunity to restore the charge lost by the pace output capacitorgradually over time up to a maximum interval equal to the paced cardiaccycle interval.

A switched capacitor pacing supply is used to restore charge to thepacing output capacitor between output pace pulses. TheSwitched-Capacitor pacing supply is provided with a multiplicity ofprogrammable voltage multiplier settings (e.g. 0.5x, 0.66x, 1.0x, 1.5x,2.0x, 3.0x), and a multiplicity of programmable operating frequencies(e.g. f=1/(n*0.5 mS)=15.6 Hz, 31.25 Hz, 62.5 Hz, 125 Hz, 250 Hz, 500 Hz,500 Hz, 1000 Hz, 2000 Hz, for n={128,64,32,16,8,4,2,1}).

The cardiac device and the method of use minimizes battery chargeconsumption by selecting the minimum voltage multiplier setting, and theminimum switched capacitor pacing supply operating frequency consistentwith restoring requisite charge to the pace output capacitor in timebefore the next cardiac pace output pulse.

A switched capacitor pacing supply Multiplier, and Clock FrequencyControl Algorithm are used to set the operating frequency and multiplierfrequency. First the pacing supply operating frequency is set to themaximum rate. The voltage multiplier setting, Kx, is set to the minimumsetting that is greater than Vpace/Vstop; this is the multiplier settingrequired to recharge the pacing supply cap at end-of-life batteryresistance and terminal voltage.

Decrease Kx, and measure the time it takes to recharge the pacing supplycapacitor from a paced event or alternately the charging rate to verifythat the pacing supply capacitor is recharged within the cardiac pacingcycle interval. Continue to decrease Kx until the charge time becomesgreater than the cardiac cycle interval. Restore the Kx setting to thatjust prior to that test cycle for which charge time exceeds the cardiaccycle interval.

Once the minimum Kx setting is determined, reduce the pacing supplyoperating clock in order that charge is extracted from the battery atthe minimum rate required to recharge the pacing supply fully by thenext commanded output pace. The following benefits are achieved by usingthe minimum operating frequency, which minimizes peak recharge current:

Battery output voltage transients are minimized thereby reducing thebattery decoupling capacitor value.

Battery energy loss across its internal output impedance is minimized.For example, consider the battery energy loss for a Lithium Iodine withbattery output resistance of 1 Kohm in recharging the pace outputcapacitance 4.0 uC at battery output current of 450 uA (battery terminalvoltage=2.35V stop limit) in 8.8 mS compared to providing the same 4.0uC but at an average current of 4 uA over a 1 second cardiac cycleinterval. The “quick” recharge will dissipate 1.78 uJ while the slowrecharge will dissipate only 0.016 uJ. The fast recharge case accountsfor about 5% of the total energy of the pacemaker per cardiac cycleassuming a typical total pacemaker operating of 13 uA, a battery voltageof 2.8V, 36 uJ per 1 second cardiac cycle.

Minimizing the operating frequency and transient peak current may alsoreduce the potential for Li-Iodine charge loss due to trapped chargeassociated with high operating currents. This is a phenomenon which iswell documented and understood for Li-Iodine chemistry wherebysignificant amounts of charge won't be extracted from the cell if the DCoperating current is too high. Under high operating current conditionscharge is extracted from the cell so fast that some charge becomestrapped within the cell and thereby not all of the charge capacity isavailable. For example, while a Li-Iodine cell may provide all of itsrated capacity at a DC current load of 20 uA, it may only be able tosupply<=80% of its capacity at 400 uA DC load. The effects of transientload current on reducing charge availability will certainly be lessenedby the presence and the low pass filtering effect of the batterydecoupling capacitor but to the authors knowledge has not widely beenstudied or been determined by industry.

The next step in the algorithm may then be to reduce the pacing supplyoperating clock from maximum using consecutively smaller discretefrequency settings ( e.g. f=1/(n*0.5 mS)=2000 Hz, 1000 Hz, 500 Hz, 250Hz, 125 Hz, 62.5 Hz, 31.25 Hz, 15.6 Hz, for n={1,2,4,8,16,32,64,128})until the charge time measurement exceeds the cardiac cycle interval.

A further refinement of the pacing supply operating frequency control isidentified within this patent which accounts for the exponentialcharging current that results when operating a switched-C pacing supplyat a fixed frequency to charge the pace output storage capacitor. When aswitched-C pacing supply is operated at a fixed frequency, f, its firstorder electrical behavior is that of a resistor of a value of R=1/Cf.The value C depends on the charge transfer capacitor value as well asthe fill and dump phase switch configuration, while f is simply thefrequency at which fill and dump cycles occur per second. Although theminimum fixed frequency, f, selection criteria as determined above willprovide the minimum number of charge transfer cycles required torecharge the pacing supply output capacitor within the cardiac pacecycle, the charge removed from the battery on each charge transfer cycleexponentially diminishes along the sequence of consecutive cycles as thepace output capacitor is charged. The rate of pacing supply chargeextracted from the battery can be “leveled” further by varying the timeinterval between individual charge transfer cycles so that theiroccurrence is the inverse of the exponentially decaying chargetransferred on each of the cycles. This improvement to the inventionrequires a variable clock frequency, where frequency increases towardthe end of the charging interval (cardiac cycle) inverse toexponentially diminishing charge transferred on each cycle. Frequencycontrol could be accomplished in a number of ways. For example, theembedded microcontroller, or state machine, could:

Count the total number of fill-dump cycles N needed to recharge thepacing capacitor.

Determine the paced cardiac cycle interval, Tcyc.

Measure the amount of discharge of the pace storage capacitor followingthe pace, ΔVTotal.

Measure the increase in voltage added to the pacing supply outputcapacitor, ΔVI, after each charge transfer cycle.

Load a presettable delay counter for the next charge transfer cycleinterval (I), with the value

Ti =Tcyc (ΔV(I)/ΔVTotal).

This relationship modulates the delay between subsequent charge transferevents, to level and equalize the average rate of charge extraction fromthe battery throughout the time the pacing supply output capacitor ischarged by ensuring that the rate of charge transfer for each individualtransfer cycle is equal to requisite charging rate needed to rechargethe output capacitor in time for the next cardiac cycle;

ΔV(I)/T(I)=ΔVTotal/Tcyc.

Periodic assessment of the battery and charge time measurements isperformed by the pulse generator's imbedded controller frequently enoughto ensure that the pace output regulation is maintained as the batterydepletes. In addition, the pacing supply voltage multiplier setting andoperating frequency are reassessed whenever pace output parameters aremodified such as commonly occurs during initial implant, and patientfollow-up. Reassessment can be triggered during ambulatory PG operationby significant paced output and or lead impedance changes, predeterminedbattery output resistance and/or voltage monitor points, or occurrenceof the pacing supply not fully charge the pace output supply before acommanded pace output. The invention gradually increases pacing supplyoperating frequency, and multiplier setting as the battery outputimpedance increases and/or output voltage decreases as the cell isdischarged.

A cardiac pacing device is described which comprises a battery, aswitched capacitor pacing power supply comprising of one or moreinternal charge transfer capacitors, and a pace storage capacitor anelement which can discharge current to the tissue of a patient in apulsatile manner. While the invention applies to a variety of switchedpacing power supply designs, a specific example of a switched capacitorpower supply suitable for use in an implantable Brady pacemaker isdescribed to illustrate an how frequency control can be utilized toshape the battery current load.

This supply is capable of operating in both voltage attenuation andmultiplier modes while transferring charge from the battery to the pacestorage capacitor, depending upon a programmable capacitor chargetransfer switch configuration setting. The specific charge transferconfigurations can be selected from a number of well know circuittopologies.

During the filling phase, the at least two charge transfer capacitors(24, 26) and a combination of switches inside said capacitor bank areconfigured to charge the charge transfer capacitors from the battery.During an alternate output, or dump phase, the switch configuration isused to move charge from the transfer capacitors the pacing supplyoutput pacing capacitor (28). This process continues until the pacingsupply output capacitor is charged to the desired pace output voltage(Vset) as determined by the Vset voltage comparator (18). The operatingfrequency of the charge transfer capacitor switches is varied over theuseful life of the battery in order to control impedance presented tothe battery terminals so that the battery pacing load current isapproximately the average pacing supply charge required to replenish thepace output capacitor divided by the cardiac cycle. Operating frequencyis adjusted so that charge is slowly extracted from the battery at arelatively uniform rate over the entire cardiac cycle. Viewed anotherway, the operating frequency of the switched-C pacing is controlled sothat the effective load impedance presented to the battery by the pacingsupply is high at the beginning of the battery's life when the battery'soutput resistance is low. As the battery discharges over life and itsoutput resistance increases the pacing supplies operating frequency isincreased in order to compensate, lowering the pacing supply impedancepresented to the battery so that output pacing capacitor may be chargedin time for the next pace command while keeping the battery load currentconstant. In the pacing device, the switching frequency rate isdetermined by a reading of the rate of charging of a storage capacitorwith said pacing device or alternately measuring the overall charge timeof the pace output capacitor.

The switching frequency rate is preferably determined by measuring thecharge time of the pace output storage capacitor. The pacing device,preferably in response to the reading of output capacitor charge time,increases the pacing supply operating frequency is increased as thelength of time to charge said storage capacitor increases.

Processes of this invention also include a process for increasing thelife of a battery in a cardiac pacing device comprising the steps ofmonitoring a feature within the cardiac pacing device which indicatesthe relative strength of the battery compared to beginning of lifeconditions, as said monitoring shows a decrease in said relativestrength of the battery compared to beginning of life conditions, andthen modifying the voltage multiplier setting which vary the filling andoutput phases of the charge transfer.

Another process for charging a storage capacitor in a cardiac pacingdevice and then discharging the storage capacitor to effect a pacecomprises the steps of:

a. monitoring a feature within the cardiac pacing device which indicatesthe relative strength of a battery compared to a defined initial batterystrength,

b. as said monitoring shows a decrease in said relative strength of thebattery compared to said initial strength battery strength, selecting analternate voltage multiplier setting by modifying the charge transfercapacitor configuration during the filling and output phase in saidcardiac pacing device which are used to store charge in a power supplyfor a storage capacitor at a frequency of switching, and

c. increasing the frequency of switching at which the at least twocharge transfer capacitors are alternatively reconfigured betweenfilling and output phases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified circuit diagram of the system of the presentinvention.

FIG. 2 shows the current and voltage relationship of the input andoutput terminals of the pacing supply.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention describes a process and apparatus which provides ameans for charging a pacemaker output capacitor for use in pacingdevices where pulsed outputs must be supplied by the device. Thecharging circuit for the output capacitor uses a variable frequencycapacitive charge pump. The variable frequency in the charge pump isparticularly effective where the chemistry of the battery (e.g., a LiIbattery or LiI-type battery) displays series impedance increases asenergy is depleted from the battery cell. The process of the presentinvention converts a fixed battery voltage (e.g., such as the 2.8 Vtypical of a LiI battery) into a variable output voltage which is storedin an output capacitor. The output voltage may be programmed at whateverlevel is required by the pacing device, but for a Brady pacemaker isusually within the range of 0.5 to 7.0 Volts. The preferred circuitcomprises at least two capacitors (e.g., banks of 2 to 10 or morecapacitors may be used) and a combination of switches to accomplish thevoltage conversion in the present invention. The capacitors and switchesare configured within the pacing power supply so that the capacitors arealternatively configured in a filling configuration and an output “dump”configuration, to effect charge transfer from the battery to the paceoutput. For example, two charge transfer capacitors can be used to forma voltage doubler ( Kx=2.0). In such a design, the charge transfercapacitors are each connected in parallel across the battery outputterminals during the filling phase and charged to battery potential (e.g 2.8V). During the dump phase, the charge transfer capacitors aredisconnected from the battery and connected in series with the outputcapacitor, under steady state conditions the pacing supply outputcapacitor is charged to 2x battery terminal voltage ( e.g. 5.6V ). Thereis preferably further control circuitry which is able to modify thefilling and output phases of this charging process so that amultiplicity of voltage multiplier settings are attainable. The outputcapacitor charge voltage can be varied at any time from, for example,0.5 to 5.0 volts. Switching of the configuration of the capacitorsbetween the filling and output configurations is performed atcontrolled, relatively high frequency so that the output capacitor maystill be charge at a rapid rate. Typically the rate of charging may bebetween 400 milliseconds (corresponding to 185 ppm) and 2 seconds(corresponding to 30 ppm) depending upon the desired pace output rate.One aspect of the present invention is that the switching frequencybetween the filling and output charge transfer configurations can be,and preferably is, varied over the useful life of the battery which isused to power the pacing device.

The cardiac pacing device may, for example, comprise:

a) a battery,

b) a discrete time (i.e., its operation is not continuous, but episodic,or its operation is defined and/or authorized at discrete points oftime), switched capacitor pacing power supply comprising an internalcharge transfer capacitor bank comprising at least two capacitors,

c) a pace output capacitor, and

d) a switch that can discharge current to tissue of a patient.

The cardiac pacing device may be constructed whereby the switchedcapacitor pacing power supply has more than one programmable voltagemultiplier settings, and more than one programmable operating frequency.The cardiac pacing device may also have the switched pacing power supplyis connected to a memory controlling voltage multiplier settings andoperating frequency so that said voltage multiplier settings and saidoperating frequency are varied automatically by the device over theuseful life of the battery.

The functional operation of the variable frequency device and itsbenefits can be understood from the following description. Thecombination of a) switched transfer capacitor value and b) the switchingfrequency presents to the system an equivalent series resistance betweenthe pacing device battery and the output capacitor. By varying thefrequency, this equivalent series resistance will in turn vary as well.At the beginning of battery life (BOL), the battery cell impedance willbe relatively low. Under this condition, the equivalent seriesresistance of the charge circuit should be selected to optimize bothcharge time and battery current drain for the BOL conditions.Specifically, this series convertor resistance should be low enough toprovide an adequately short charge time consistent with the present pacepulse output rate of the pacemaker (e.g., 400 ms (185 ppm) to 2.0 second(30 ppm), but the series resistance should be high enough to precludeexaggeratedly fast charge times that would resulting in excessive peakbattery currents. For example, at a pace output setting of 3.5V, 0.4 mSpulse width, 500 Ohm lead, a 10 uF output capacitor will be dischargedto about 3.25V after the pace pulse is delivered. A switched capacitorsupply operating at 2 Khz, using qty 2, 0.1 uF charge transfercapacitors, in a 1.5x multiplier setting will recharge the outputcapacitor from 3.2V to 3.5V in as few as qty 6 cycles, or about 3 mSecs,transferring approximately 4 uC within 3 mSec from the battery terminalresulting in a peak battery current during the recharge interval of 1.3mA. If this latter event should happen, the too high current drain willcause fluctuations in battery cell voltage to occur. In an extreme case,battery voltage can fluctuate to levels that are low enough to cause areset of the pacing device to be initiated depending upon the size ofthe battery decoupling capacitor. These battery voltage fluctuationscould interfere with various device features such as sense amplifierperformance, estimated replacement time (ERT) measurement, and otherperformance functions of the device. The process may therefore operatewherein the frequency of switching and/or a functional setting on amultiplier (after this self-applied analysis and comparison is made)effects a battery current profile that increases battery longevity froma standard frequency (e.g., a default or preprogrammed frequency) ofswitching and/or standard (e.g., a default or preprogrammed setting onthe multiplier) functional setting on a multiplier.

As the LiI battery capacity is depleted over time, the series batteryimpedance increases. As a result, the optimal converter seriesresistance decreases correspondingly. The system of the presentinvention addresses this decrease in converter series resistance byincreasing the converter frequency. As a result, the optimal capacitorcharge time, as well as the optimal battery current drain may bemaintained. The variation in converter frequency may be performedautomatically by the pacemaker based upon parameters preset within theprogramming of the device such as the last measured time charge, anaverage of recent past time charges, and the like. This variation in theswitching frequency of the charge circuit is at least one of theimportant differences over prior art pacemakers which fix a frequencyover the life of the device. A fixed frequency will not be extended tothe level capable in the practice of the present invention. As apractical matter the use of fixed voltage multiplier setting mapping topace amplitude setting, also means that current drain for charging theoutput capacitors is relatively high at BOL to enable an adequatelyshort charge time to be maintained at the End Of Life (EOL) periodnearing depletion of the battery. If a fixed mapping between paceamplitude and voltage multiplier settings is used throughout life, asignificant charge inefficiency applies to the top of the pace amplituderange defined for each multiplier setting because it will often be thecase that a lower multiplier setting could be used at beginning throughmid-life to achieve adequate charge times. FIG. 2 illustrates thatbattery charge comes at a significant cost at increasing higher pacingsupply voltage multiplier settings.

Specifically, the present invention improves upon the efficiency andeven the performance under extreme conditions of previous systems byaltering the pacing supply voltage multiplier setting and switchingfrequency from relatively low frequencies at BOL to provide relativelyhigh converter series resistance, to relatively high switchingfrequencies at EOL to provide relatively low converter switchingresistance. This results in limiting the peak current drain at BOL,preventing fluctuations in voltage, while maintaining an adequatelyshort charge time at EOL. This combination assists in increasing theeffective life of the battery cell, and extends the period whereininvasive replacement of the battery cell must take place.

Charging System Overview

The system of FIG. 1 charges the load 28 capacitance using switchingpower supply 20 and battery 36. As shown in FIG. 1, battery 36 ismodeled using idealized elements, such as ideal voltage supply 4,internal impedance 6 and internal capacitance 8. Internal impedance 6varies primarily as a function battery charge depletion but can exhibitadditional increases in apparent output impedance under the influence ofthe temporary increases in current drawn from battery 36 and the powerdrawn from battery 36. For example, internal impedance 6 may increasetemporarily when large currents are demanded from battery 36. Internalimpedance 6 also may increase as a function of power delivered bybattery 36. An increase in internal impedance 6 results in a lowering ofoutput voltage at node 10.

The battery output voltage at node 10, Vbatt will fall when charging theload 28 using switching power supply 20. In one embodiment, the systemensures that this drop in voltage does not fall below a Vstop voltagereference level due to charging of the load 28. For example, in FIG. 1,an input clock signal, CLK, to switching power supply 20 is used toactivate or deactivate the switching power supply 20. Comparator 12monitors the voltage at node 10, Vbatt, and compares it with the voltagereference Vstop to deactivate the switching power supply should thevoltage at node 10 fall below Vstop. The deactivation is accomplished bya signal to logic “AND” gate 16. Other circuit configurations arepossible without departing from the present invention. Vstop isgenerally selected to ensure that the battery voltage at node 10 doesnot fall below a predetermined safe operating voltage. This prevents thebattery voltage from falling below the minimum required to properlyoperate the electronics in the system and prevents the system fromentering a “reset” condition during charging cycles.

Another control feature found in the system of FIG. 2 is the Vsetcontrol, which inhibits charging of load 28 once the voltage across thepace output capacitor at node 30 has reached a desired value. In thecircuit of FIG. 1, a voltage from node 30 is sampled and compared to adesired Vset voltage by comparator 18. Once the sampled voltage reachesthe desired Vset voltage, a signal is sent to logic “AND” gate 16 toinhibit further charging of the load 28 by switching power supply 20. Inone embodiment a voltage divider network is used between node 30 andcomparator 18, since the voltages sampled may be large relative to thepower supply used to power comparator 18. In the embodiment where avoltage divider is used, the Vset voltage may also be set using theknown voltage divider ratio.

The clock controller or frequency controller 34 is used to control thefrequency of the switched-capacitor pacing supply (e.g., 2 kHz/n). It isa programmable frequency divider which divides an input clock frequency,e.g. 2 Khz, by a presettable integer count, n, loaded by the internalpacemaker controller to produce the switched capacitor operatingfrequency=2 Khz/n. For example, the controller can command amultiplicity of switched capacitors pacing supply operating frequencies,f=1/(n*0.5 mS)=15.6 Hz, 31.25 Hz, 62.5 Hz, 125 Hz, 250 Hz, 500 Hz, 500Hz, 1000 Hz, 2000 Hz, for n={128,64,32,16,8,4,2,1}. The internalpacemaker controller controls the pacing supply voltage multiplicationsetting through a digital control bus (37) to specify the multiplicityof possible settings ( e.g. 0.5x, 0.66x, 1.0x, 1.5x, 2.0x, 3.0x), theswitched-capacitor pacing supply (20) takes this bus as an input andmodifies the charge transfer capacitor (24, 26) filling and output phaseswitch topology in order to achieve the voltage multiplicationspecified.

A schematic of one type of pacing system 2 according to the presentinvention is shown in FIG. 1. The battery 4 and the battery resistance 6lead past the battery capacitor 8 (Cb). At ajunction 10 the electricalpath divides into two paths. The first path leads to the battery voltageVstop comparator 12 and the second path provides power to the internalcircuitry represented in FIG. 1 simply as a current load 14. The secondpath continues on into the switched capacitor power supply 20, which isshown as having two charge transfer capacitors 24 26. The first paththrough the battery voltage Vstop comparator 12 leads is used to enableand disable switched capacitor 16. The pacing supply digital clock isenabled as long the battery voltage, Vbatt, remains above the Vstoplimit and is applied to the power supply 20 along path 22. The chargefrom the battery 36, is transferred by the pacing power supply 20 passesthrough the charge transfer capacitors 24, 26 junction 30 through line32 and is used to charge the pacing output capacitor 28. As noted, theportion of the pacing unit 2 above line A-B is a conventional design. Inthe practice of the present invention, a frequency controlling circuitryor digital clock divider 34 controls the operating clock frequency line22, of the switched-capacitor power supply 20 through line 22. Thefrequency controlling circuitry 34 has the capability of having thefrequency varied without invasion of any patient in whom the pacingsystem 2 has been implanted. Variation in the frequency may be effectedby any convenient mechanism, particularly programs within the pacingdevice 2 in combination with a conventional clock divider circuitwhereby a high frequency clock reference is digitally divided by n byemploying a programmable counter. For example, the embedded pacemakercontroller may use the frequency controlling circuitry 34 to effect a)gradually changes to the frequency over a time period in which thebattery charge is expected to diminish under programmed usage, b) atiming function which gradually changes the frequency at specific timeintervals after the pacing device 2 has been activated in a patient, orc) samples or reads the actual current/voltage/battery resistance andthis information is used by a logic function (hardware, software, chip,switch, etc.) to alter the frequency of the current. It is preferredthat the frequency be altered by a program within the device which doesnot need any outside monitoring or reprogramming, which would indicate apreference for the latter three types of systems.

FIG. 2 shows the current and voltage relationship of the input terminal102 and output terminal 104 of the pacing supply 100. The voltagemultiplication factor, kx, is based upon the electronic structure andnumber of switching capacitors 106, 108.

What is claimed:
 1. A process for charging a storage capacitor in acardiac pacing device and then discharging the storage capacitor toeffect a pace, the process comprising: monitoring a feature within thecardiac pacing device which indicates the relative strength of a batterycompared to a defined initial battery strength; as said monitoring showsa decrease in said relative strength of the battery compared to saidinitial strength battery strength, alternately configuring at least twocapacitors in said cardiac pacing device which are used to transfercharge from the power supply to the storage capacitor at a frequency ofswitching; and increasing the frequency of switching at which the atleast two charge transfer capacitors are alternatively configured in acharge filling phase from the battery and a charge delivery phase to apace output capacitor.
 2. The process of claim 1 wherein said monitoringis done by measuring a rate of charging for at least one capacitor insaid cardiac pacing device.
 3. The process of claim 1 wherein saidfrequency of switching and/or a functional setting on a multipliereffects a battery current profile that increases battery longevity froma standard frequency of switching and/or standard functional setting ona multiplier.
 4. The process of claim 1 wherein each change in frequencyis based upon a monitoring of a property or event in a previous pace orin a previous series of paces.
 5. The process of claim 1 wherein: a) theoperating frequency is initially set to a maximum rate; b) then thevoltage multiplier setting, Kx, is set to the minimum setting that isgreater than Vpace/Vstop; c) Kx is then decreased, and time is measuredfor how long it takes to recharge the pacing supply capacitor from apaced event or the charging rate to verify that the pacing supplycapacitor is recharged within the cardiac pacing cycle interval; d) Kxis decreased until the charge time becomes greater than a cardiac cycleinterval; and e) the Kx setting is restored to that just prior to a) forwhich charge time exceeds the cardiac cycle interval.
 6. The process ofclaim 5 wherein once the minimum Kx setting is determined, the pacingsupply operating clock is reduced so that charge is extracted from thebattery at the minimum rate required to recharge pacing supply fully bya next commanded output pace.
 7. A method for charging a storagecapacitor in a cardiac pacing device and then discharging the storagecapacitor to effect a pace, the process comprising: a) during a fillingphase, at least two charge transfer capacitors and a combination ofswitches inside said capacitor bank are configured to charge the chargetransfer capacitors from the battery; b) during a dump phase, the switchconfiguration is used to move charge from the transfer capacitors to apacing supply output capacitor (28); c) with a) and b) continuing untilthe pacing supply output capacitor is charged to a desired pace outputvoltage (Vset) as determined by a Vset voltage comparator; d) theoperating frequency of the charge transfer capacitor switches is variedover the useful life of the battery in order to control impedancepresented to the battery terminals so that the battery pacing loadcurrent is approximately the average pacing supply charge required toreplenish the pacing supply output capacitor divided by the cardiaccycle.
 8. The method of claim 7 wherein operating frequency is adjustedso that charge is slowly extracted from the battery at a relativelyuniform rate over the entire cardiac cycle.
 9. The method of claim 8wherein as the battery discharges over life and its output resistanceincreases, the pacing supply operating frequency is increased, loweringpacing supply impedance presented to the battery so that pacing supplyoutput capacitor may be charged in time for the next pace command whilekeeping the battery load current constant.
 10. A process for charging astorage capacitor in a cardiac pacing device and then discharging thestorage capacitor to effect a pace, the process comprising: monitoring afeature within the cardiac pacing device which indicates the relativestrength of a battery compared to a defined initial battery strength,wherein said monitoring is done by measuring a rate of charging for atleast one capacitor in said cardiac pacing device; as said monitoringshows a decrease in said relative strength of the battery compared tosaid initial strength battery strength, alternately configuring at leasttwo capacitors in said cardiac pacing device which are used to transfercharge from the power supply to the storage capacitor at a frequency ofswitching; and increasing the frequency of switching at which the atleast two charge transfer capacitors are alternatively configured in acharge filling phase from the battery and a charge delivery phase to apace output capacitor, wherein each change in frequency is based upon amonitoring of a property or event in a previous pace or in a previousseries of paces.